Defects on a steel pipe used as various kinds of pipes, critical parts of machinery, line pipes, and the like reduce its strength, toughness, and fatigue properties, and defect inspections are thus performed in manufacturing a steel pipe. Non-destructive inspections such as eddy current testing, magnetic leakage flux testing, and ultrasonic testing and eye inspection are common techniques of defect inspection. However, these techniques are less applicable to high temperature materials and are thus applied for a steel pipe, which includes a seamless steel pipe manufactured by hot rolling and a butt-welded steel pipe, after cooling down the materials. In manufacturing such kinds of steel pipes, a surface defect caused by rolling is detected only after a large number of steel pipes have undergone the rolling processing, which results in manufacturing a large number of non-conforming products. A technique is thus desired that can inspect the surface when the steel pipe is hot.
As a technique of inspecting a hot surface, for example, Iwai et al., “On-line inspection techniques for surface detects of hot slabs”, Iron and Steel 70 (9), pp. 1181-1187 (1984) describes techniques that use an image (a self-luminous image) formed by using self-luminescence, an image formed by using an external light source, a thermal image formed by induction heating and techniques of using laser scanning, eddy current, and the like. Shiraiwa et al., “Color TV system for inspection of hot slabs under slabbing”, Iron and Steel 64 (13), pp. 2020-2025 (1978) describes a technique of inspecting the surface of a slab using a self-luminous image. The surface temperature of a slab differs depending on whether a surface defect exists thereon, and this technique detects a surface defect based on a difference in luminance between self-luminous images.
Use of a self-luminous image for surface inspection of a steel pipe, however, has the following problem. The rolling of seamless steel pipes uses a plurality of rolls disposed in the circumferential direction. The amount of heat removed by contact of each roll is different, which results in uneven temperature on the surface of the steel pipe after the rolling. Furthermore, variations in the size of a steel pipe affect the uneven temperature on the surface in removing heat by contact with the rolls. Primary scale generated in heating and secondary scale generated in rolling may attach to the surface of a steel pipe in a “spotty” manner. Such a disturbance changes the surface temperature in patterns and causes variations in the luminance of a self-luminous image. It is thus difficult to determine whether the change in the surface temperature is caused by the disturbance or by a defect on the surface.
It could therefore be helpful to provide a surface inspection method, a surface inspection device, a manufacturing system, and a method of identifying a defect formed area of a steel pipe to detect a surface defect on the steel pipe when the steel pipe is hot using a self-luminous image.